1) Field of the Invention
This invention relates to a B.sub.4 C ceramic material useful as an abrasive material, a wear- and abrasion-resistant material, a high-temperature material, an energy-related material or the like, and especially to improvements in its toughness and strength.
2) Description of the Related Art
B.sub.4 C ceramics are recently attracting interests as engineering ceramics for high-temperature structural members. To promote the use of B.sub.4 C ceramics in fields where materials are required to exhibit high reliability and stability under severe conditions like car parts and aerospace materials, it is indispensable to improve their fracture toughness to overcome their brittleness and also to improve their high-temperature strength. As a technique for improved fracture toughness, it has heretofore been known, for example, to distribute silicon carbide whiskers in a silicon nitride matrix as disclosed in Japanese Patent Publication No. 62-265 173. This technique appears to bring about such improved fracture toughness because, upon fracture, whiskers may probably deflect propagating cracks and/or whiskers may perhaps be pulled out or tangled together. Fracture toughness can therefore be improved by the combined use of whiskers. The whiskers so incorporated however have a length on the order of 1-10 .mu.m. In practice, it is difficult to completely avoid their aggregation by any mechanical means. Accordingly, such whisker aggregates, as coarse grains, tend to act as starting points of fracture, leading to a reduction in strength.
Therefore, it has heretofore been the practice either to add whisker or to subject B.sub.4 C to grain growth so as to improve the fracture toughness owing to the inclusion of coarse grains. As a result, defects are indeed enlarged, thereby lowering the strength. It has thus been difficult to improve both strength and toughness.